Method for removal of gas from molten metal during continuous casting



Aug. 20, 1968 J D. GRICOL 3,397,733

' METHOD FOR REMOVAL OF GAS FROM MOLTEN METAL DURING CONTINUOUS CASTINGFiled Dec. 13, 1965 FIG. 2

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I IIWIIMI a a z mfig E 2% i-mim-fiimim-nmn United States Patent3,397,733 METHOD FOR REMOVAL OF GAS FROM MGLTEN METAL DURING CONTINUOUSCASTING Joseph D. Gricol, Flat Rock, Mich, assignor, by mesneassignments, to Concast Incorporated, New York, N.Y.,

a corporation of Delaware Filed Dec. 13, 1965, Ser. No. 513,244 1 Claim.(Cl. 1647l) ABSTRACT OF THE DISCLOSURE Vibrations are applied to a slabof cast metal emerging from a continuous-casting mold at a positionalong the slab at which the interior of the slab is still molten. Thefrequency of the vibrations are adjusted to be in resonance with theportion of solidified metal skin around the slab between the position atwhich the vibrations are applied and the point at which the skin isformed in the mold; the amplitude of the vibrations are adjusted toagitate the surface of the molten metal in the mold.

This invention relates to the continuous casting of steel and, moreparticularly, to the continous casting of rimming steel.

In continuous casting of steel, molten steel is poured into are upperend of a mold open at its bottom. The mold is cooled with a liquid, suchas water, circulated in the mold walls. The molten metal in contact withthe mold wall is cooled or chilled and solidifies. This solidified metalforms a skin or sheath around the molten metal.

The cast metal leaves the bottom of the mold in a continuous ribbon orcast. The ultimate length of the continuous cast, of course, dependsupon the amount of molten metal poured into the top, or tundish, of themold. Casts of indefinite length can thus be continuously cast.

The length of the mold employed in continuous casting of steel isrelatively short. As the molten metal moves through the mold the metalin contact with the mold wall is cooled and solidifies. At the pointwhere the cast slab leaves the mold there is only a relatively thin skinor sheath of solid metal at the exterior of the slab. The remainder ofthe metal in the slab is molten and is contained in the slab by the skinof solid metal. As the slab moves away from the bottom of the mold themolten metal continues to cool and solidify, from the exterior of theslab inwardly, increasing the thickness of the skin or sheath until,finally the entire cast slab is solid. The I point where completesolidification of the slab occurs is at a substantial distance below thebottom of the mold, as much as thirty-five feet, or more, from the moldbottom in some operations. As the slab emerges from the mold, the slabis supported by rollers engaging the slab from its opposite sides. Someof these rollers are driven and control the speed of the slab.

The melt for continuous casting of rimming steel is prepared at atemperature of about 2950 to 3060 F. The melt is poured into a ladle andfrom the ladle into the tundish. From the tundish the molten metal isdelivered to the continuous casting mold at a temperature of about 2800to 2875 F.

To obtain the desired rimming action, a regulated amount of uncombinedoxygen is left in the molten metal at the time of pouring in thecontinuous casting of rimming steel. This uncombined oxygen, as themetal solidifies, forms gas in the molten metal. This gas agitates themolten metal, causing the metal to circulate in a pattern. Thiscirculation produces the desired rimming action. It is, of course,desirable that this gas, as it forms in the molten metal, move to thetop of the molten metal ice and escape. Any gas present in the moltenmetal at the time the metal solidifies is trapped and forms undesirablevoids or occlusions in the slab. These voids or occlusions, when theyoccur, result in flaws and defects when the continuously cast slab islater rolled.

A substantial portion of the cooling and solidification of the metal incontinuous casting occurs below the mold as the cast slab moves awayfrom the mold bottom. The uncombined oxygen still in solution in themolten metal is carried along with the molten metal in the slab untilthe molten metal is cooled. Thus, at a point in the cast substantiallydistant from the point Where the molten metal is poured into the mold,gas bubbles are still forming.

One of the objects of the instant invention is to provide an improvedmethod for continuous casting of rimming steel.

A further object is to provide apparatus for carrying out such method.

A further object is to provide such a method and apparatus whereindissolved oxygen which, when it reacts to form a gas, will escape fromthe cast.

Still a further object is to provide continuous castings of rimmingsteel which can be rolled into relatively thin sheets without fiaws ordefects on the surfaces of the sheets.

These and other objects will be more apparent from the followingdescription and attached drawings in which:

FIG. 1 illustrates schematically apparatus of the invention forcontinuously casting steel, viewed from one edge of the slab being cast;and

FIG. 2 is a view similar to FIG. 1 but viewed from one side of the slabwith parts of the apparatus omitted.

Many attempts have heretofore been made to apply vibratory forces tocontinuous casting operations. For the most part, these prior attemptshave involved vibration of the mold or the slab after the metal in theslab is solidified. Such prior attempts employed rugged, expensiveequipment, required specially designed molds and had many disadvantages.In addition, they were not effective in removing gas occlusions andvoids in the continuously cast slab.

In the instant invention, vibration is applied to the cast slab while asubstantial portion of the slab is molten. The shell of solid metalaround the molten metal is vibrated so that the tapered walls of solidmetal agitate the molten metal within the slab. The agitation causes thegases to separate from the solidfying surface, move upward through themolten metal in the slab center, and escape. In addition, the vibrationagitates the molten metal in the mold, causes the molten metal to movein a pattern and improves the rimming action.

In carrying out the invention, vibrators are positioned at the oppositeedges of the slab at a point below the mold where the center of the slabis still molten. These vibrators are in contact with the opposite edgesof the slab and impart vibration to the solid metal around the exteriorof the slab. From the point where this vibration force is applied,upward of the slub and into the mold area, the thickness of the solidmetal wall around the molten metal progressively decreases. Thisprogressive decrease in solid metal thickness has been discovered, inthe instant invention, to result in a tuning action. The upwardlyextending arms of solid metal act as tuning forks, or a tuned horn, withthe center filled with liquid molten metal.

The vibration input, at the point where the vibratory force is appliedto the slab, is relatively low. However, the progressive decrease inthickness of the solid metal from the input point to the point where themolten metal is initially cooled and the skin or shell of solid metalbegins to form, results in an increase in vibration amplitude from thepoint of input to the point in the mold where the walls of solid metalinitially form. This increase in amplitude correspondingly increases theagitation of the molten metal. Thus, gas bubbles are released from thesolidifying surface and move upwardly through the molten metal to escapeinto the atmosphere.

Referring now to the attached drawings where the apparatus of theinstant invention is schematically illustrated on a conventionalcontinuous steel casting machine, the apparatus includes tundish 2, openat its top and having a nozzle 4 at its bottom. A mold 6 is disposedbelow tundish 2, in position to receive molten metal from nozzle 4. Mold6 is reciprocated, in conventional manner, parallel to the direction ofmovement of the cast slab by conventional means not shown. In itsdownward travel, mold 6 is driven at a speed slightly faster than thatof the cast being continuously formed and is moved upward at a higherspeed, all in accordance with conventional continuous casting of steelprocedures.

Mold 6 is open at its top and bottom. Vertical beams 8, 10 are disposedin parallel position below the open bottom of mold 6. Each beam 8, 10 isprovided with a plurality of rollers 12, the rollers being relativelyclosely spaced vertically along the beams on the facing sides of beams8, 10.

Vibrator 14 is mounted on support 16 intermediate beams 8, 10. A shoe 18having, at its opposite vertical ends, outwardly turned portions 20, 22,is connected to, and driven by, vibrator 14. Preferably, two vibratorsare employed, one at either edge of the cast slab.

In operation, molten metal is poured from a source, not shown, into theupper open end of tundish 2. The molten metal flows through nozzle 4into the open upper end of mold 6. The flow of metal through nozzle 4 isregulated, in known manner, to maintain the level of molten metal in themold cavity at the required height.

Liquid, such as water, is circulated in the walls of mold 6 to cool thewalls. As the molten metal passes through mold 6, the metal in contactwith the walls is cooled and the metal solidifies, forming a shell orsheath of solid metal around the molten metal in the center of the moldcavity. Thus, while in the mold cavity, solid metal b commences to formas a shell or sheath around molten metal a.

When the cast slab emerges from the bottom of the mold 6, the shell orsheath of solid metal b is relatively thin. As the slab moves away fromthe bottom of mold 6, cooling continues. This cooling may besupplemented by spraying a liquid, such as water, against the slab asthe continuously forming slab travels away from the bottom of mold 6.Cooling progresses from the exterior of the slab inwardly. Additionalmolten metal a solidifies on the interior of the shell or sheath ofsolid metal b, thickening the shell or sheath of solid metal until,finally, at a substantial distance below the mold, all of the metal inthe cast is solidified.

The cast, as it emerges from the mold, comes into contact with rollers12 carried on beams 8, 10. Rollers 12 maintain the slab in alignment andsupport the slab during cooling. These rollers may be idle rollers ormay be driven. Preferably, some of the rollers are idle and others aredriven. The drive on the driven rollers regulates the speed at which thecontinuous cast is formed.

Vibrators 14, through shoes 18, vibrate the cast, from its oppositeedges, in the direction of the arrows in FIG. 2. The vibrators arelocated on the apparatus below the mold bottom where the wall of solidmetal in the cast is relatively thick but the center portion of the castslab is of molten metal. The vibratory force applied to the cast,through the solid metal b, agitates the molten metal a in the cast andcauses gas bubbles, as they form on solidification of the metal, toseparate from the solidifying surface and move upwardly through themolten metal in a direction opposite to the travel of the continuouscast.

The vibration input may be tuned to the resonant frequency of the solidmetal b in the walls of the solid metal shell above the vibrators. Theamplitude and frequency required for resonance will, of course, varywith the size of the cast, the amount of cooling and the like, but canbe readily determined by observing the activity of the molten metal inthe mold. The amplitude and frequency of the vibrators are adjusted tobring the surface of the molten metal in the mold to the desiredagitation. The amplitude of vibration of the solid metal shell increasesfrom the point of vibration input as the thickness of the solid metaldecreases. Thus the agitation imparted to molten metal a by vibration ofsolid metal b increases from the vibration input point upwardly into themold cavity. By regulating the vibration input, agitation of the moltenmetal in the mold can be controlled. This control of the agitationpermits the molten metal to form a rim of solid metal around theexterior of the continuously cast slab and facilitate the movement ofimpurities away from the surfaces of the slab in addition to decreasingvoids and occlusions near the slab surfaces.

The capacity of the vibrator will, of course, depend on the size of thesteel slab to be continuously cast and, preferably, should be of a typethat can be regulated. An air-powered vibrator having a capacity of100,000 pounds thrust and frequency up to 3,000 cycles/min. has beenfound particularly suited for the production of continuously castrimming steel slabs having a thickness of ten inches and a width offifty-two inches.

The terms and expressions which have been employed are used as terms ofdescription and not of limitation, and there is no intention, in the useof such terms and expressions, of excluding any equivalents of thefeatures shown and described or portions thereof, but it is recognizedthat various modifications are possible within the scope of theinvention claimed.

What is claimed is:

1. In a method for continuously casting a slab of rimming steel whereinmolten steel is poured in one end of an open-ended continuous-castingmold that is cooled to form a shell of solidified metal around the metaltherein, a continuous slab of the metal having a solidified shell and astill molten interior is withdrawn from the other end of the mold, andthe slab issuing from the mold is thereafter continuously cooled tosolidify the slab all the way through, the improvement comprising:removing gas from the molten metal in said slab by applying a vibratoryforce directly to the side of the slab after it emerges from the moldand at a position along the slab at which the interior of the slab isstill molten with said molten interior extending up into the moltenmetal in the mold; adjusting the frequency of the vibratory force toapply it at the resonant frequency of the skin of solidified metal whichis between said location and the point of formation of the skin in themold; and adjusting the amplitude of the vibratory force to a levelsufiicient to agitate the surface of the molten metal in the mold.

References Cited J. SPENCER OVERHOLSER, Primary Examiner.

R. S. ANNEAR, Assistant Examiner.

